1. Field of the Invention
Embodiments of the present invention generally relate to laser combiners, and more specifically, to all-fiber devices that combine optical laser power from multiple separate sources such as lasers or amplifiers.
2. Description of Related Art
In applications involving high power industrial lasers, fiber-based systems are increasingly attractive. Typically constructed from Ytterbium-doped gain fibers emitting in the 1060 nm wavelength band, fiber lasers with single-moded beam quality are limited to powers in the 200 to few kW range. For certain applications such as cutting and welding of metals, higher levels into the multi-kW range are needed. Currently, this high power is obtained by combining the power from several aforementioned laser fibers into a single fiber, which necessarily exhibits multimoded properties due to the incoherence derived from the separate sources. Still, this technique has been successful in coupling 1-10 kWs of power into a 50-100 um core fiber with numerical aperture less than or equal to 0.22 NA. In many material processing situations, it is desirable for the beam to be multimoded rather than singlemode since a multimode beam can have a more uniform spatial intensity profile.
To perform beam combining, current techniques invoke bulk optical lenses to collimate the fiber inputs and steering mirrors to direct multiple beams into a common fiber core. As can be expected, issues occur when trying to use coated glass components and moving mechanical systems with high powers. These details can create prohibitive cost and maintenance problems.
An all-fiber approach would alleviate these problems, since it is a sealed unit device with no internal moving parts and does not require adjustment or alignment. While multiple fibers can be bundled together and coupled to an output fiber with a large, multimode core, the packing density of the cores in the bundle is typically very low. For example, in a typical singlemode fiber, the core covers less than 1% of the total fiber cross-section. Bundling such fibers will result in significant reduction in the brightness of the optical source when coupled into the common output fiber. While tapering of the fiber bundle will reduce the separation between cores and increase the packing density, there is a limit to the degree of tapering which may be used before the optical power in the individual cores either leaks out of the core or begins to couple into adjacent cores. Taper ratios of only about 3 are possible before this occurs, allowing only a modest improvement in brightness through tapering. There exists a need in the art for an all-fiber approach which can provide close spacing or high density of multiple cores. High packing density can enable additional applications which are not possible with sparse core spacing.
In addition to combining of multiple individual beams, there are applications which benefit from splitting of a single beam into multiple beams. This allows a common source such as a high power solid state laser to be used at multiple endpoints. One application of such a device is to illuminate an area from multiple directions using a common source, such as for medical therapeutics in cancer treatment. In such situations, again, the individual cores in the bundle should have as tight a packing density as possible to capture as much optical energy as possible from the singe multimode core at the tapered end of the device.